1. Field of the Disclosure
The disclosure generally relates to phase interpolators, and more specifically to techniques to reduce I/O count and redundant coding in a phase interpolator.
2. Related Art
Recent digital communication standards impose demanding jitter tolerance that requires the use of phase interpolators with high rotation rates and high resolutions. Conventional phase interpolators, including digital control, may include digital control words utilizing thermometer coding schemes addressing each phase individually. Such coding schemes may include Gray coding between adjacent phases allowing single bit changes between adjacent phases, thus allowing high rate phase rotation while avoiding discontinuities due to race hazards within digital control circuits, especially in pipelined and look-ahead applications. Unfortunately, systems addressing individual phases suffer from excessive control word length as the number of available phases increase to provide high resolution, driving high I/O count requirements on phase interpolators, impacting circuit layout, packaging, signal integrity, and PCB layout. Thus, phase interpolators that employ optimized coding schemes that reduce I/O count and control word length, while still providing high resolution, and allowing high rate phase rotation, may provide significant advantages.
To reduce I/O count and control word length, some exemplary systems employ quadrant-based phase interpolator control schemes that partition a full 360 degrees of phase rotation into four 90-degree quadrants to reduce I/O count and control word length from a directly addressed scheme. Two bits address each quadrant and a plurality of thermometer encoded bits, depending on the resolution, address intermediate phases and relative phase strengths within each of the four quadrants. Thus, in an exemplary embodiment wherein each quadrant contains four bits of resolution, a total of six bits encode a full 360 degrees of phase rotation, including 16 separate phases. This scheme is a significant improvement over, for example, a directly addressable scheme that would require a full 16 thermometer encoded bits to address 16 separate phases. A plurality of on-off amplifiers, addressed by the thermometer encoded bits within a given quadrant, amplify a selected two 90-degree adjacent phases at 0, 90, 180, and 270 degrees. Unfortunately, this scheme suffers from redundant encoding at 0, 90, 180, and 270 degrees, requiring an additional coding step during phase rotation, introducing delay and race hazards into control circuitry that may give rise to undesirable and unpredictable discontinuities.
Embodiments of the disclosure will now be described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number.